Resin-encapsulated packages can be fabricated in large quantities at a low cost, and thus are most widely used as packages of semiconductor elements for general use.
However, an encapsulation resin has low permittivity, and thus flexibility is low in designing characteristic impedance of lead terminals through which a high frequency signal is input to and output from a semiconductor element. There are few examples in which resign-encapsulated packages are used when a high frequency signal of hundreds of MHz or more is involved. In particular, since a high-power semiconductor element has a large chip size, the input/output impedance of the semiconductor element itself is low, and a mismatch is likely to occur between the input/output impedance and the impedance of a load circuit including characteristic impedance of a lead terminal of a package.
Patent Literature (PTL) 1 discloses, as a technique of designing characteristic impedance of lead terminals, a method in which ground leads are provided on both sides of each signal lead, the widths of the signal leads and the ground leads, and the intervals therebetween are adjusted, and the signal leads are caused to function as coplanar distributed constant lines.
Furthermore, PTL 2 discloses a package structure for causing lead terminals to function as microstrip distributed constant lines.
FIG. 8 is a cross-sectional view illustrating a structure of a mold package disclosed in PTL 2. In a conventional mold package 500 illustrated in the drawing, a first resin 503 is molded first to incorporate first external leads 502 connected to electrodes of a semiconductor element 501. Then, second external leads 504 which form metal layers are placed on the resin molded first, and a second resin 505 is molded second to incorporate the first external leads 502 and the second external leads 504, together with the resin molded first. Specifically, in the mold package 500, the first external leads 502 inside the molded resin, which serve as inner leads for signals, face the second external leads 504 that form ground metal layers, thereby achieving lead terminals which function as microstrip distributed constant lines.